1. Field of the Invention
This invention relates generally to electrostatic precipitators (ESPs) used to precipitate particulate matter from exhaust gases onto collection substrates by electrostatic charge, and more particularly to a laminar flow, wet membrane collecting electrode ESP.
2. Description of the Related Art
Industrial ESPs are used in coal-fired power plants, the cement industry, mineral ore processing and many other industries to remove particulate matter from a gas stream. ESPs are particularly well suited for high efficiency removal of very fine particles from a gas stream. Specially designed ESPs have attained particle collection efficiencies as high as 99.9%. However, conventional ESP collection efficiencies are at their lowest values for fine particle sizes between 0.1-1.0 xcexcm. Additionally, conventional ESPs cannot address the problem of gaseous emissions or gas-to-particle conversion.
In 1997 the Environmental Protection Agency (EPA) proposed new air quality standards for fine particulate matter. The focus of the regulations is the emissions of fine particulate, i.e., particles below 2.5 xcexcm in aerodynamic diameter (PM2.5). These fine particulates are a health danger, because the human body cannot prevent these small particles from entering the respiratory tract and lungs.
In a typical conventional ESP, vertical wire electrodes are placed in the midsection of a channel formed between vertical parallel collector substrates. The heavy, typically steel, plates are suspended from a support structure that is anchored to an external framework. Commonly, ten or more of the single precipitation channels constitute a field. Industrial precipitators have three or more fields in series. An example of such a structure is shown and described in U.S. Pat. Nos. 4,276,056, 4,321,067, 4,239,514, 4,058,377, and 4,035,886, which are incorporated herein by reference.
A DC voltage of about 50 kV is applied between the wire electrodes (discharging electrodes) and the grounded substrate collector plates (collecting electrodes), inducing a corona discharge between them. A small fraction of ions, which migrate from the wires toward the plates, attach to the dust particles in the exhaust gas flowing between the plates. These particles are then forced by the electric field to migrate toward, and collect on, the plates where a dust layer is formed.
In dry ESPs, the dust layer is periodically removed from dry ESPs by hammers imparting sharp blows to the edges of the plates, typically referred to as xe2x80x9crappingxe2x80x9d the plates. When ESPs are rapped, the dust layer is supposed to drop vertically downward from the plates due to a shear force between the plate and the parallel dust layer. The compressive loading in this so-called normal-rapping mode generates fast propagating stress waves, along and across the plate, that are manifested in large lateral amplitude displacements of the plates in the direction perpendicular to the plane of the plate.
Pasic et al., in U.S. Pat. No. 6,231,643, which is incorporated herein by reference, first disclosed the principle of using a membrane as a collecting electrode in a dry or a wet ESP in order to avoid the large deflection of the electrode due to rapping. However, the turbulent flow of gases around the membrane electrodes prevented substantial collection of acid aerosols and fine particulate.
Control of fine particulate and acid aerosols are of vital importance to the burning of coal that is inherently high in sulfur. The higher the sulfur content, the higher the SO3 content, and therefore, the more likely that sulfuric acid aerosol formation will occur, especially in units that use selective catalytic reduction (SCR) for NOx control. The resulting opacity from the acid aerosols has caused plants to reduce their output during these exceedances.
Current particulate control devices, such as precipitators and bag filters, have problems with collection of fine particulate and acid gases, which later form aerosols known as secondary PM 2.5. Effective collection of submicron particles with bag filters is inherently difficult and creates unacceptably large pressure drops across the filter. ESPs have a particularly difficult time collecting particles in the size range of 0.1-1.0 xcexcm, because the two dominant modes of particle charging, field and diffusion, go through combined minimums in this size range, and because particle charge depends on the strength of the electric field. In dry precipitators corona current and electric field strength is suppressed as the electrically resistive ash layer builds on the collecting surfaces. This effect can even lead to formation of back corona in dry precipitators.
The control of NOx emissions using selective catalytic reduction (SCR) technology is likely to aggravate SO3 emissions at existing coal-fired power plants. Several plants with SCRs have experienced catalytic oxidation of SO2 to SO3. SO3 vapor, in combination with water vapor, converts to gaseous sulfuric acid. When SO3 vapor reaches saturation upon cooling or in contact with water, aerosols of fine sulfuric acid mist are formed. Most of these aerosols reside in a particle size range between 0.1 and 0.5 xcexcm. At these sub-micron particle sizes the light scattering phenomenon is also at a maximum. This will result in a highly visible plume even for relatively small amounts of sulfuric acid aerosols. The resulting opacity can lead to temporary de-ratings of units, costing the plant potential sales.
A conventional ESP operates with turbulent flow in the gas channels. Because of the turbulent eddies, 100% collection efficiency is approached only asymptotically and cannot be attained no matter how large the precipitator. One theory that has been commercialized for dry precipitators to address their inherent problems with fine particulate collection is the use of laminar flow in precipitation. In laminar flow the flow streamlines are parallel and in the direction of flow, and therefore, there are no turbulent forces causing particles, especially fine particles, near the collecting surface to be blown back into the central flow region. Therefore, 100% collection efficiency is possible in laminar flow.
To create laminar flow, as is known, the Reynolds number (Re) must first be less than 2300 where   Re  =                    V        gas            ⁢              ρ        g            ⁢              D        h                    μ      g      
where Dh is the hydraulic diameter defined by       D    h    =            2      ⁢              (                  Δ          ⁢                      xe2x80x83                    ⁢          x                )            ⁢      H                      Δ        ⁢                  xe2x80x83                ⁢        x            +      H      
where xcex94x is plate spacing and H is the height of the collection electrode.
Reducing gas velocity to attain Re less than 2300 has been attainable since the first precipitator was built. However, laminar flow in ESPs is still prevented by the cross flow due to corona wind. The cross-flow caused by corona wind continuously disrupts the laminar flow conditions and creates a rebound effect from the solid collecting surfaces.
In 1998 Environmental Elements Corporation (EEC) overcame the problem of cross-flow caused by corona wind by using planar discharge electrodes with lower voltage, that are positioned much closer together than in conventional ESPs and have virtually no current flow. The idea behind a laminar flow precipitator is to vastly reduce the distance between the collection plates and as such, lower the Reynolds number below 2300, the generally accepted condition for transition to turbulent flow. Further, the plates must be smooth, as surface imperfections create disruptions of the boundary layer or induce turbulence outright. Both factors are employed to limit formation of turbulent flow.
The EEC device relies on upstream, turbulent flow electrostatic precipitator fields to remove 95+% of particulate in the gas stream and to charge all remaining particles before the particles reach the laminar region. However, the dry laminar precipitator in the EEC device fails to permanently collect particles. This is because, although the EEC device eliminates corona wind, it also eliminates the current flow that serves, in conventional ESPs, as the main adhesive force for cold-side precipitator ash. The current keeps a flow of charged particles striking the electrode to pin other particles onto the collector. In a dry precipitator, little collection can be done without corona to further charge and hold particles already collected in place by striking them with other charged particles. So while the EEC dry laminar precipitator was able to collect fine particulate with increased efficiency, the majority of particles were rapidly re-entrained due to the moving gas stream and the lack of current flow.
In the process of initial collection on the laminar EEC device, smaller particles temporarily attach to the collecting surfaces, and, through collision, the particles connect to each other, forming larger particles due to agglomeration. Without current flow, and thus with low adhesive forces, the larger particles re-entrain into the gas flow. A downstream conventional, turbulent precipitator field collects the larger particles, which become easier to collect due to their increased size. The invention has now been marketed as the Fine Particulate Agglomerator (FPA) and is discussed in U.S. Pat. No. 5,759,240 to Becker.
While dry electrostatic precipitation has been used in laminar arrangements, such as EEC""s collector, it cannot be used collect acid aerosols unless the gas stream temperature is reduced below the acid dew point. This creates numerous problems in a dry environment, such as corrosion and wet-dry interfacings. Furthermore, another ESP is necessary downstream from the EEC device to collect the agglomerated particles. This consumes valuable, and possibly unavailable, space.
The invention is an electrostatic precipitator for collecting matter from a flowing gas stream. The precipitator comprises at least one, and preferably a plurality of, substantially planar discharge electrodes disposed in the gas stream substantially parallel to the gas stream flow direction. The discharge electrodes have an electrical charge.
At least one, and preferably a plurality of, substantially planar collecting electrodes is disposed in the gas stream substantially parallel to the discharge electrodes, and alternated between the discharge electrodes. The collecting electrodes and the discharge electrodes are in such close proximity that the gas stream between the electrodes flows in a substantially laminar manner.
The collecting electrodes are made of a substantially water-saturated porous membrane having a water-wetted, exterior surface. The collecting electrodes have an electrical charge that is opposite in polarity to the electrical charge of the discharge electrodes. This thereby forms an electric field between the electrodes to cause particulate matter from the gas stream to be precipitated onto the collecting electrode during operation. The water serves as both a conductor and a trap for the matter that is collected.
In a preferred embodiment, at least one, and preferably a plurality of, charging electrodes are disposed in the gas stream upstream of the collecting electrode for charging some of the matter in the gas stream before the matter flows between the collecting and discharge electrodes.
The invention is capable of removing acid aerosols, soot, and ultrafine particles with no complicated scraping hardware, special seals, or secondary collection equipment. The ash layer in the laminar wet ESP does not create an insulating effect in the water on the membrane, and therefore there is no corona current and electric field strength suppression. The use of continuously wetted collecting electrodes also minimizes the formation of back corona. This is because the wet ESP has constantly wetted and cleaned surfaces, and because water that contains ions, and is uniformly distributed via capillary transport, is an excellent conductor. Therefore, the wet precipitator can deliver far greater energizing power due to higher voltages and field strengths, and can effectively charge even submicron particles. Testing by the inventors of aerosol and particulate collection using a bench-scale laminar wet precipitator has indicated that both re-entrainment of collected aerosols and particulates is eliminated, but also that uniform field strengths of 400 kV/m are possible without the onset of corona if the correct electrode configuration and materials are used. These field strengths are equal to, or higher than, the typical turbulent dry precipitator.
The potential of membrane-based wet precipitation to control acid aerosols, condensed hydrocarbons and soot, and fine and ultra-fine particles is very good. The continual wetting action via capillary flow and flow along the outer surface causes water to act as both the collecting electrode and the cleaning mechanism to prevent back-corona and loss of collection efficiency. In addition, the use of water as a collector eliminates re-entrainment because the collected particle xe2x80x9csticks toxe2x80x9d or is absorbed by the water with forces much stronger than the transport effects of bulk gas flow. Once the particle is collected, it will not be re-entrained as seen in dry precipitators
By using water, two main advantages are gained. First, because of the high degree of adhesion between water and solid particles, any particle reaching the collecting surface will be held, without re-entrainment, and carried away with the water. The water in the laminar wet ESP collects and removes particles collected at near 100% efficiency through attainment of laminar flow in a very high voltage field. Second, because of the large volume of water in this field and the close proximity of the electrodes, the gas stream temperatures will be reduced to below the dew point for most of the gases, condensing acid gases and creating acid aerosols. These aerosols can then be collected in the water on the collecting membranes, which may be in one of numerous configurations, but must be wet.
Because the invention is a wet system, potential applications include, but are not limited to vertical flow uses, such as immediately downstream of a wet scrubbing (for SO2 control) system to act to remove acid aerosol and water mist, or as a last field in a horizontal flow (hybrid) precipitator, where the laminar wet precipitator acts as a polishing unit, or as an entirely separate polishing unit that follows some other bulk particulate removal device.